1. Field of the Invention
This invention relates to a fuel injection system for direct injection into the combustion chamber of an internal combustion engine through a valve controlled port.
The characteristics of the spray of the fuel droplets issuing from a nozzle into a combustion chamber have major effects on the efficiency of the burning of the fuel, which in turn affects the stability of the operation of the engine, the engine fuel efficiency and the composition of the engine exhaust gases. To optimise these effects, particularly in a spark ignited engine, the desirable characteristics of the spray pattern of the fuel issuing from the nozzle include small fuel droplet size, controlled penetration of the fuel spray into the chamber, and at least at low engine loads, a relatively contained ignitable cloud of fuel vapour in the vicinity of the spark plug.
Some known injection nozzles, used for the delivery of fuel directly into the combustion chamber of an engine, are of the poppet valve type, from which the fuel usually issues in the form of a conical spray, with the fuel droplets forming a generally continuous conical wall extending from the peripheral edge of the poppet valve.
The nature of the shape of the fuel spray is dependent on a number of factors including the geometry of the port and valve constituting the nozzle, especially the surfaces of the port and valve immediately downstream of the seat where the port and valve engage to seal when the nozzle is closed. Once a nozzle geometry has been selected to give the required spray pattern, relatively minor departures from that geometry can impair the engine performance. In particular the build-up of solid combustion products on surfaces over which the fuel flows is detrimental to the desired spray pattern and correct performance of the nozzle.
When build-up does occur it is normally not of a uniform nature around the peripheral extent of the surface of the nozzle over which the fuel flows, thus severely disturbing the symmetry of the fuel spray.
2. Description of the Relate Art
It is known that build up on the surface of the injection nozzle, such as carbon deposits, can be removed or the formation thereof controlled, if the relevant surface of the nozzle is exposed to temperature conditions sufficient to burn off the build up of contaminants such as carbon. However, the cooling effects of the adjacent walls of the combustion chamber, which are frequently water or air cooled, and the cooling effect of the fuel being delivered through the nozzle, are such that under normal operating conditions, the relevant surfaces of the nozzle do not reach a sufficiently high temperature to effect removal of the contaminants that may build up on the surfaces of the nozzle.
There have previously been proposals to construct the delivery area of the nozzle so as to restrict the heat flow path therefrom in the endeavour to raise the temperature of the relevant surfaces where contaminants may build up. A typical example of such nozzle constructions is to be found in U.S. Pat. No. 4,817,873. These proposals have met with varying degrees of success, but have the major problem that the useful life of the nozzle is seriously reduced as a result of the relevant areas being maintained for long periods at the higher temperature necessary to effect removal of the contaminants.
There is disclosed in U.S. Pat. No. 4,395,025 a direct injected internal combustion engine wherein a mechanically operated injector nozzle is cyclically opened and closed to permit delivery of fuel to the combustion chamber. The delivery of the fuel is effected by a high pressure charge of combustion gases from the combustion chamber which is delivered into a fuel chamber in the injector body by maintaining the injector nozzle open for an extended period after completion of the injection of the fuel into the combustion chamber. Upon the initial opening of the injector nozzle, the pressure of the gas in the fuel chamber is sufficiently above the compression pressure in the combustion chamber to discharge the vapourised fuel from the fuel chamber into the combustion chamber. Ignition and combustion of the fuel subsequently commences and the injector nozzle is maintained in the open position well into the combustion period so that the hot high pressure gases generated by combustion will flow into the fuel chamber and be subsequently trapped therein on closing of the injector nozzle. The trapped hot gases are at a pressure sufficient to effect injection of the fuel during the next engine cycle, where such injection is timed to occur at a point in the compression stroke when the pressure in the combustion chamber is below the pressure of the gas trapped in the fuel chamber.
In this proposal, a charge of high temperature, high pressure combustion gas is delivered to the fuel chamber each cycle o the engine and fuel is constantly delivered into the injector chamber through a permanently open metering orifice. The metering of the fuel is effected by the fixed size orifice and a variable pressure pump supplying fuel to that orifice.
It is to be noted that the proposal in U.S. Pat. No. 4,359,025 does not deal with the problem of the build up of deposits in the injector nozzle, which adversely influence the spray pattern of the fuel delivered to the nozzle, and is primarily directed to mixing the fuel with high temperature gas to effect vapourisation thereof prior to delivery through the nozzle to the combustion chamber. There is no discussion in this disclosure of the problem arising from the build up of solid contaminants in the nozzle or a solution to this problem. It is considered that in the light of experience, the high temperature conditions in the fuel chamber, generated by the presence of combustion products therein together with unburnt fuel would lead to the generation of solid and/or gum deposits which would seriously impair the operation of the injector. In particular, it is considered that gum deposits would be generated in the upper pressure chamber wherein a piston operates to effect closure of the injector nozzle. This could lead to sticking of the piston and hence potential ineffective closing of the injector nozzle. Further it is believed that there would be a build up of deposits in fuel metering orifice and the passage leading therefrom, which would adversely effect the accuracy of the fuel metering system.